1. Field of the Invention
This invention relates to diagnostic ultrasonic imaging in general and in particular to the display of ultrasonic imaging data relating to the elastic properties of scanned tissue.
2. Description of the Related Art
Ultrasonic elasticity imaging is a technique whose use has become more widespread as the processing power of ultrasonic imaging systems has grown enough to handle the often heavy computational loads required by the technique. As is well known, elasticity imaging is based on the same principle as the manual palpation that has been used by physicians for millennia: Tumors and other xe2x80x9clumpsxe2x80x9d can often be detected simply by compressing the surrounding tissue. Even to this day, for example, most breast cancers are discovered by self-examination using manual palpation, and physicians still rely on palpation to detect potential tumors of the liver and prostate.
The principle of manual palpation is well known and is based on the property that if a compressive force is applied to an elastic body, then it will deform. If a relatively stiffer, that is, less elastic, inclusion is located within a region of the body, then a constant compressive displacement will deform the region above the stiff object more than the adjacent regions. Because tissues are elastic, the more they are deformed, the greater counter force they generate; in other words, large stress leads to large deformation. If a diagnostician applies the pressure with her fingers, then she will often be able to feel the stress distribution above the palpated region. To sum up the procedure, if one presses on body tissue, then one can often feel xe2x80x9clumps.xe2x80x9d
Ultrasonic elasticity imaging emulates manual palpation, but has several advantages. One advantage is that it can provide information about tissue elasticity as deep as the ultrasound can penetrate, which is usually deeper than a physician can feel with her fingers. Another advantage is that ultrasonic elasticity imaging has relatively high sensitivity, although resolution and sensitivity are reduced for deeper inclusions. Yet another advantage is that ultrasonic elasticity imaging can provide a 2-D cross sectional view of the elastic properties of the object. Still another advantage is that information about tissue elasticity obtained using ultrasound can be more easily quantified, and can be stored for later analysis.
Using ultrasound to create an image of the displacement or strain profile (which is related to elasticity) within a region of insonification often reveals structures that are invisible or hard-to-detect in a conventional B-mode image either because of noise, or because the acoustic impedance of the internal structure is not different enough from the surrounding tissue to provide adequate B-mode contrast. In many cases, however, the elastic properties of such structures are so different from those of the surrounding tissue that an image of the strain profile will show the structure clearly, or at least much more clearly than a B-mode image.
Because strain is a function of the derivative of displacement, at least two B-mode images are required for each estimate of strain. Accordingly, in ultrasound elasticity imaging, two B-mode frames are generated while the clinician uses the ultrasound transducer (or, in some cases, an external mechanism) to vary the stress on the imaged portion of the patient""s body, for example, through cyclical compression and decompression. The 2-D displacement function is then estimated by comparing the scans at different stress levels. Object strain and/or elastic constants can then be estimated from the estimated displacement function.
Many different methods have been proposed to create an estimate of strain within a 2-D scan region once two B-mode frames are available for comparison and analysis. The algorithms underlying these known methods typically rely on cross-correlation, echo data modeling, block matching, direct strain estimation using adaptive local stretching algorithm, and the analysis of a deformable model. Once an ultrasound system generates information about the strain distribution within the scanned portion of the patient""s body, it must be displayed in some way. The conventional way to display elasticity data is by converting the strain values to corresponding gray-scale values and then to present these in the same way as any B-mode image.
Even the most accurate algorithm for estimating a strain profile within a region of a patient""s body is all but useless, however, if the data are not presented to a physician in such a way that he can clearly see the inclusions the scan detected. Because elasticity imaging is essentially a function of a difference between B-mode images, equivalent to a first derivative of local (often, pixel-to-pixel) displacement, it is particularly sensitive to noise. Moreover, several cycles of compression and relaxation are usually gone through during the course of a typical elasticity scan. At each time of transition, or whenever else the physician stops changing the degree of compression, there will be little or no change in displacement between temporally adjacent B-mode frames. At these times, most of what the physician will see on the display will be either noise or artifact. This can be very distracting, and may even prevent the user from making sense of the display when it is showing valuable elasticity information.
The distractions caused by noise and artifacts are even more pronounced in the typical gray-scale displays of elasticity data. Gray-scale images may be clear enough in conventional B-mode imaging, in which one tries to hold the transducer relatively still once a structure of interest has been acquired, but elasticity imaging typically requires the user to cyclically press and release the probe against the patient""s body. Consequently, elasticity displays are by definition dynamic, with many frames of xe2x80x9cnoisexe2x80x9d at transitions between compression and decompression.
One way to avoid this is for the various frames of elasticity data to be stored either digitally, on tape, or on some other recording medium, for later display in a xe2x80x9ccinexe2x80x9d mode. Even then, however, the user must decide which frames are showing diagnostically useful information and which should be ignored.
What is needed is therefore an improved display of elasticity data. Ideally, the display system should also make it easier for the user to identify and concentrate on display frames that have a relatively high signal-to-noise ratio. This invention provides such a display.
The invention provides an ultrasonic imaging method and system implementation that have various aspects. Common to all embodiments of the invention is that a region of interest (ROI) of a body is scanned a plurality of times using an ultrasound transducer in order to acquire a first and second set of intensity values. Each intensity value in each set represents an imaging property of a respective portion of the ROI, such as echo signal strength.
According to one aspect of the preferred embodiment of the invention, a gray-scale (B-mode) representation of the first set of intensity values is generated in the conventional manner. A set of elasticity valuesxe2x80x94an elasticity data setxe2x80x94is also calculated as a function of differences (corresponding, for example, to displacement) between corresponding intensity values in the first and second sets of intensity values. A color representation of the elasticity data set is then also generated by color-coding the elasticity values. Both the gray-scale (B-mode) and color (elasticity) representations are then displayed simultaneously as a single, registered, overlaid display.
The overlaid display is preferably generated as a linear combination of the gray-scale representation and the color representation, for example, with the visibility of the gray-scale and color representations, respectively, being functions of a transmission coefficient. The transmission coefficient may be fixed, set automatically, or made user-adjustable. The overlay may also be generated as a non-linear combination, for example, the product, of the two representations.
According to another aspect of the preferred embodiment of the invention, the display of the elasticity data set is adaptively persisted. The degree of persistence applied to a current elasticity display is preferably dependent either on a quality factor indicating the quality the current elasticity frame, or on one or more previous elasticity frames, or on both.
According to yet another aspect of the preferred embodiment of the invention, a measure of quality of each of a series of displayed frames is calculated. A graphical representation of this quality measure is then preferably displayed along with the current frame. By either maneuvering a displayed marker or by stepping through display frames (as in conventional cine), the user can select a particular frame for display, for example, a frame with high indicated quality, guided by the displayed graphical representation of the quality measures for the different frames.